1. Field of the Invention
The present invention relates to a motor-driven power steering system which employs an electric motor for generating assistive steering power that is applied to a steering mechanism to reduce a steering force to be manually applied, and a method of controlling such a motor-driven power steering system.
2. Description of the Relevant Art
One conventional motor-driven power steering system for use on an automotive vehicle is disclosed in Japanese Laid-Open patent publication No. 59-130780 (corresponding to British patent application No. 2 135 642A), for example. In the disclosed power steering system, the steering torque applied by a steering wheel to a steering mechanism is detected by a steering torque detecting means, and the speed of travel of the automotive vehicle is detected by a vehicle speed detecting means. Based on the detected signals, the electric motor for producing assistive steering power is controlled to produce an assistive steering torque commensurate with the vehicle speed.
Generally, an electric motor produces a low output torque and rotates at a high speed, whereas the assistive power source for a motor-driven power steering system is required to produce a high output torque and rotate at a low speed. To meet such a torque/speed requirement, the above conventional power steering system employs a gear train as a speed reducer for applying the motor power therethrough to the steering mechanism in reducing the manually imposed steering force.
However, the conventional motor-driven power steering system does not have a means for detecting the steering speed and a means for detecting returning motion of the steering mechanism after the steering wheel has been turned while the vehicle is running. Since the motor is controlled without signals from such detecting means, the stability of the vehicle as it is running at a high speed is rather poor when the steering mechanism returns to its neutral position.
More specifically, when the steering mechanism returns to the neutral position, it is forcibly moved back to the neutral position under reactive forces from the tires due primarily to the caster trail. Inasmuch the speed reducer is rotated from its output side at this time, the motor is rotated at an increased speed, and the moment of inertia of the motor in the steering mechanism is of a very large value as it is proportional to the square of the speed reduction ratio of the speed reducer. While the vehicle is running at a low speed and the reactive forces from the tires are small, the inertial moment of the motor assists in quickly returning the steering mechanism which is therefore automotically moved back rapidly while being released from the driver's hands. At high speeds of travel, however, the reactive forces of the tires are so large that the steering mechanism may move too far beyond its neutral position and then back again. Such reciprocating motion is repeated until the neutral position is reached. Therefore, the time required for the steering mechanism to settle to the neutral position is relatively long, making the vehicle unstable while the steering mechanism is returning during high-speed travel of the vehicle.